1. Field of the Invention
The invention relates generally to fluid bed biological reactors including bed particles supporting biological material, and more particularly to control apparatus incorporated into fluid bed biological reactors to remove excess biological material therefrom.
2. Reference to Prior Art
Fluid bed reactors are used in the biological processing of waste water. A fluid bed biological reactor includes a reactor tank containing a media bed of particulate solids, such as sand or activated carbon, which serve as a substrate for microorganisms. The waste water is conducted upwardly through the reactor tank at a velocity sufficient to fluidize the media bed, and the microorganisms are nourished under aerobic conditions by impurities in the upflow to process the waste water. The principles of operation of fluid bed biological reactors are provided in the following U.S. patents: U.S. Pat. No. 4,202,774 issued May 13, 1980 to Kos; U.S. Pat. No. 4,182,675 issued Jan. 8, 1980 to Jeris; U.S. Pat. Nos. 4,009,105, 4,009,099 and 4,009,098, all issued Feb. 22, 1977 also to Jeris; and U.S. Pat. No. 3,956,129 issued May 11, 1976 and U.S. Pat. No. 3,846,289 issued Nov. 5, 1974, both to Jeris et al.
Excess biological material (i.e., biological material in excess of that needed for normal reactor operation) must be removed from the reactor to insure its proper and efficient operation, as discussed in U.S. Pat. No. 4,177,144 issued Dec. 4, 1979 to Hickey et al. In the above-mentioned Jeris and Jeris et al. patents, the growth of biological material in the media bed is monitored as a function of bed expansion by a bed level sensing device. When the media bed reaches a predetermined height to activate the sensing device, the bed particles are abraded with a mechanical stirrer to strip excess biological material therefrom. The partially stripped bed particles settle back into the media bed and the excess biological material is carried away in the reactor effluent stream. This is undesirable since sludge dewatering equipment, etc. must in some instances be added to the system downstream of the reactor to separate and remove the excess biological material from the treated effluent stream.
U.S. Pat. No. 4,177,144 illustrates an excess biological growth control system including an agitator arrangement having a mixing blade positioned in a separator column that is emersed in an effluent head above the media bed. Alternatively, the agitator arrangement can include a shearing pump positioned outside of the separator column and either inside or outside of the reactor, as illustrated in U.S. Pat. No. 4,250,033 issued Feb. 10, 1981. In that agitator arrangement liquid and coated bed particles are withdrawn from the separator column and passed through the pump to shear excess biological material from the bed particles before reintroduction into the separator column. Disadvantages associated with this shearing arrangement include exterior piping (if the pump is outside the reactor) and the possible need to disconnect the agitator arrangement from the separator column for maintenance purposes, for example.
Once sheared from the bed particles, excess biological material is confined within the separator column and withdrawn therefrom at a point below the surface of the effluent head. The use of the separator column is intended to prevent sheared biological material from entering the reactor effluent stream. However, in the foregoing arrangement there is a potential for accumulation of biological material in the separator column that can inhibit operation of the control system. A further disadvantage of the control system is the high water content of the sludge (i.e., waste biological material and liquid) withdrawn from the separator column, this high water content necessitating the inclusion of significant dewatering equipment into the control system.